Facebook has 21m regular users via mobile phones

"Facebook COO Sheryl Sandberg said that 20.8 million people now use Facebook on mobile devices, and that mobile users are far more active than those who don’t use it on their mobile phone. In November, Facebook reported 15 milllion mobile users. The social-networking site has about 150 million users overall."
Source: Facebook COO Sheryl Sandberg, talking at the Palm Keynote at CES 2009, as reported by mocoNews, 8th January 2009.
This is an increase of nearly 6m in 2 months